Sine of the Times

Today's advanced adjustable-speed drives (also called variable-frequency inverters) and motors increasingly can be found in production processes in manufacturing plants, machinery and robotics, elevator systems in high rise buildings, commercial air conditioning systems, and washing machines. Despite indisputable advantages of adjustable-speed drives (ASD), there are a few less-mentioned drawb...

By Udo Leonhard Thiel and Mike Griese

10/01/2005

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Today's advanced adjustable-speed drives (also called variable-frequency inverters) and motors increasingly can be found in production processes in manufacturing plants, machinery and robotics, elevator systems in high rise buildings, commercial air conditioning systems, and washing machines.

Despite indisputable advantages of adjustable-speed drives (ASD), there are a few less-mentioned drawbacks.

Inside ASDs

Requirements of modern drive systems have led to major developments in ASD technologies. Today, low-cost, three-phase asynchronous motors can easily satisfy the most demanding automation application. The governing variable is the electronically modified motor voltage. The sinusoidal supply voltage is first converted into dc in the drive's intermediate circuit (dc bus). A semiconductor switch (insulated gated bipolar transistor; IGBT) is used to generate the necessary output voltage corresponding to motor specifications and current requirements.

Starting from the zero crossing of the sine curve, there are initially very narrow pulse bursts with low energy content. Up to the peak of the sine curve, the impulse length increases. From the peak downwards, the impulse duration decreases down to zero transition again. The same processes occur in the negative half wave, only the voltage polarity changes. Variation in the voltage time (PWM) area controlled by the processor is used to control the motor's rotating field frequency, torque, acceleration, and deceleration. The first graph shows typical form of ASD output voltage.

New applications

Use of progressive ASDs opens up many new areas of application for electromotive drive technology. Several advantages account for this trend:

There also are disadvantages caused by the dv/dt characteristics of the output voltage. In some circumstances, these problems can restrict the successful use of this type of drive technology. Disadvantages follow:

Typical form of adjustable-speed drive output voltage is shown, which comprises packages of energy with a switching frequency of 2-16 kHz.

Electromagnetic capability problems are conducted and radiated. Separation of the control voltage into individual square-wave packets necessary for this technology is achieved by extremely fast-switching semiconductors (IGBTs), which creates a wide spectrum of high-frequency interference emissions. These are spread through radiation and conducted through any connected electrical lines. To keep these interferences within guidelines outlined by IEEE and EN (European Norm), it often is necessary to implement interference suppression by means of suitable filters.

Steep switching edges (dv/dt) reduce the operating life of the motor. Modern semiconductor switches in ASDs are characterized by extremely fast switching between 'on' and 'off' phases. This results in sudden voltage changes with edge steepness (dv/dt) of up to 10 kV/

With a sine filter connected to the drive's output, the dv/dt value is reduced to a level suitable for the motor. Sine filters also are highly recommended for older motor systems without ASDs. Limitation of the dv/dt value can also be achieved with low-inductive parallel reactors in the output circuit. In this case, however, it is particularly important to monitor for voltage increase at the motor terminals.

High over voltages from reflective wave phenomenon can cause insulation failure in the motor winding. High operating frequencies and extremely fast dv/dt require a new way of approaching motor cable length. Due to the high-frequency voltage content, motor wiring must be done according to the theory of RF emission. Behavior of a long electrical cable is directly dependent on the surge impedance of both cable ends. Voltage blocks created by the variable-frequency drive (VFD) are fed to the motor terminals as reflective waves. Reflected with different polarities at the cable ends, this can result in over voltages of up to twice the VFD output voltage. Varnish-insulated wires in the motor winding are not designed for these over voltages, leading to failure of the insulating material and motor. Sine filters can eliminate these reflective waves, reduce terminal voltage to acceptable limits, and prevent motor failure and costly system downtime.

Motor bearings can fail. Asymmetries in the construction of the motor, particularly between pairs of poles, are known to induce voltages in the motor shaft, even if the supply voltage is truly sinusoidal. These voltages cause a low-frequency current to flow to ground through the bearings. While improvements in manufacturing tolerances had largely eliminated this problem, introduction of the latest variable-speed drives with high-speed switched outputs (IGBTs) has resurrected the problem in a more acute form. Resulting erosion of the bearing races and bearing balls causes rapid deterioration of the bearing due to material removal. Bearing current reactors and sine filters are custom developed to counter these symptoms.

Causes of bearing currents are high frequency common mode voltages, created by the ASD's high frequency switching pulses. Every change in the common mode voltage induces a voltage in the motor shaft.

The voltages induced in the stator are very low in value (approximately 2-10 V, depending on motor size), but they can be discharged only through motor bearings. Thickness of the lubricating film and type of lubricant that forms a dielectric determine how often the high frequency bearing current flows and its value. Value of the intermediate circuit voltage and rate of voltage rise for the switching frequency directly influence the value of the common mode voltage and, indirectly, the bearing current value.

There are three types of bearing current: high frequency circulating currents within the rotor shaft-bearing-stator body cause the greatest number of bearing failure due to erosion; high frequency earth leakage currents (due to inadequate stator-body grounding—these can flow as HF currents through the bearing to the load machine coupled to the motor); and capacitive discharge currents (shaft voltages discharge sporadically through the dielectric).

Bearing current can be reduced. Various measures can be taken to minimize bearing currents: Bearings can be insulated (not a current industry standard nor widely adopted); bearing current reactors can smooth the common mode voltage and reduce bearing current, typically by a factor of 5-10; and sinusoidal filters can smooth the common mode voltage and reduce the symmetrical dv/dt motor load. This prevents motor insulation degradation due to partial discharges. A combination of sinusoidal filter and bearing current reactor offers optimum motor protection and greatly reduces bearing currents and sinusoidal motor voltage.

Long cables can require increased capacity. Frequency dependent capacity between ground and the phases of the motor cables increases proportionately with the length of the cable. The resulting resistance capacitor (XC) has a very low resistance for the radio-frequency components and places an extra burden on the ASD by increasing actual motor load. In extreme cases, the sum of available energy will be dissipated in the cable and the motor will stop running.

Sine filters vary in size and capabilities. These are SFB Series Sine Filters from Block USA.

Motor generates significant noise. Nonsinusoidal motor voltage can increase motor noise at a higher level than previously experienced. Radio-frequency amplitude components of the operating voltage lead to magnetostriction in motor laminations. Disharmonic, additional noises of a significant level result. These noises—unpleasant to the human ear—cannot be tolerated if multiplied. With the sine filter, the motor will operate at normal noise levels.

How a sine filter works

A sine filter uses two principles to generate the sine voltage form from the pulse-width-modulated ASD output signal.

Integrated direct axis inductance functions as a storage reactor, which transforms the magnetic energy and feeds it back into the load circuit as electrical energy. Frequency-dependent impedances XL (inductor) and XC (capacitor) operate as a first-order low-pass filter. Its frequency limit is set so that only the desired signal can pass with minimal energy loss. There is a voltage drop of typically 30 V, which will not limit high-motor-torque requirements. Radio-frequency components create a harmonic distortion of 5-8%; for peak requirements, improved versions are available.

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